Inertial measurement units generally make up a large portion of the cost of vehicles equipped with one. They provide inertial referenced velocity and attitude changes to a suite of navigation maintenance algorithms that 1) integrate them into evolving position and velocity estimates for vehicle guidance and 2) calculate pitch, yaw, and roll attitude and rate estimates for autonomous flight control.
A typical IMU mechanizes three orthogonal accelerometers and gyros. The advantage of an IMU is that it provides data from a purely internal frame of reference, requiring measurements only from its internal instrumentation and, therefore, rendering itself immune to jamming and deception. The disadvantages of IMUs are 1) their cost and 2) their inherent instrument drift, which manifests itself as an accumulating error in position, velocity, and attitude.
This combination of cost and navigation drift is especially problematic for applications where high accuracy is essential, such as missiles. GPS is a potential alternative, but it does not provide the attitude and attitude rate information required for flight control.